Nanomaterials have important potential applications in modern technologies. Depending on the number of spatial dimensions on which the material expands beyond the nanoscale, nanomaterial can be classified as zero-dimensional (nanoparticles), one-dimensional (nanorods and nanowires), and two-dimensional (nanosheets and nanometer thin films). For each of these categories of nanomaterials, the synthesis methods are different. For example, nanoparticles are often produced using the sol-gel process; nanorods are often produced using the vapor-liquid-solid (VLS) process [S. Wagner and W. C. Ellis, Applied Physics Letters, Vol. 4 (1964), 89; A. M. Morales and C. M. Lieber, Science, Vol. 279 (1998), 208; W. Lu, C. M. Lieber, Journal of Physics D: Applied Physics, Vol 39 (2006), R387, S. C. Tjong ed., Nanocrystalline Materials, Elsevier, Amsterdam, 2006, pp 95 and U.S. Pat. No. 5,897,954, U.S. Pat. No. 6,036,774, U.S. Pat. No. 6,225,198], which involves pre-deposited metal (such as gold) particles as a catalyst to induce nanorod growth; and nanometer thin films are often grown by epitaxy methods such as molecular beam epitaxy (MBE) and chemical vapor deposition (CVD). Regarding the VLS method that is commonly used for growth of nanorods and nanowires, because the pre-deposited metal catalyst can bring in an undesirable impurity for many applications (for example, gold is detrimental to silicon devices), many attempts have been tried to develop non-catalytic or self-catalytic (i.e., using the same material intended for growth as the catalyst) growth methods. Examples are U.S. Pat. No. 6,225,198, U.S. Pat. No. 6,720,240, and W. I. Park et al., Applied Physics Letters, Vol 80 (2002), 4232, L. C. Chen et al., Journal of Physics and Chemistry of Solids, Vol 62 (2001), 1567.
On the other hand, for material growth, regardless of the intended final morphological forms, a means of producing gaseous or liquid phase source material is always needed. Among the various methods of producing gaseous source materials (e.g., thermal evaporation or using chemical precursors), pulsed laser ablation (PLA) is a relatively new method. In this technique, a pulsed laser beam is focused on to a target surface to ablate materials. Typical lasers used for ablation include Q-switched Nd:YAG and excimer lasers, which can provide pulses with a pulse energy of a few hundreds of mJ and a pulse duration of a few nanoseconds. Because of the short pulse duration, the peak power is very high. Under such intense laser irradiation, the target surface material is inevitably evaporated. The resultant vapor is often ionized, appearing bright, and is thus called plume. The vapor can then be deposited onto a substrate to form new forms of materials. This constitutes the basics of the technique of pulsed laser deposition (PLD). FIG. 1 illustrates the PLD setup used in the current invention.
The most widely pursued morphological form of the deposited material using PLD is two-dimensional thin film. Nanoparticles have also been generated by supplying a background gas with a high pressure (a few Torr) during ablation to force particle nucleation in the laser-induced vapor [T. G. Dietz et al., J. Chem. Phys. 74, 6511 (1981), T. Seto et al., Nano. Lett. 1, 315 (2001), M. Hirasawa et al., Appl. Phys. Lett. 88, 093119 (2006)]. The particles can then be carried by a carrier gas to a substrate. Using metal particles as catalysts and PLA as the evaporation source, semiconductor nanorods have been produced in an essentially VLS (vapor-liquid-solid) fashion [A. M. Morales and C. M. Lieber, Science, Vol. 279 (1998), 208; W. Lu, C. M. Lieber, Journal of Physics D: Applied Physics, Vol 39 (2006), R387; S. C. Tjong ed., Nanocrystalline Materials, Elsevier, Amsterdam, 2006, pp 97].
Using very high power pulsed lasers (with pulse energy greater than 1 J/pulse), carbon nanotubes have been produced in large quantities [Alex A. Puretzky et al., Physical Review B, Vol 65 (2002), 245425].
One drawback of high power nanosecond PLD is the generation of micron scale liquid droplets, which is mainly a result of liquid splashing and collateral damage of the target near the focal spot due to the high pulse energy. These large droplets introduce undesirable inhomogeneity to the deposition, especially in thin film growth.
In comparison to nanosecond pulsed lasers, ultrafast pulsed lasers (with a pulse duration ranging from sub-picosecond to a few picoseocnds), when used for ablation and material deposition, have the advantage of a much lower ablation threshold [by an order of magnitude, [D. Du et al., Applied Physics Letters, Vol 64 (1994), 3071 and U.S. Pat. No. RE 37,585]] and a resultant potential of droplet-free growth [E. G. Gamaly et al., Journal of Applied Physics, Vol 95 (2004), 2250]. The very low ablation threshold is in principle because of two reasons. First, the extremely short pulse duration means a much higher peak power. Second, because the pulse duration is shorter than the typical time scales of electron-lattice interaction and heat conduction, the heat-affected zone in ultrafast pulsed laser ablation does not extend beyond the laser focal spot. This further increases the energy density at the focal spot. Because of the much reduced size of the melt pool and collateral damage, generation of large liquid droplets can in principle be suppressed. These characteristics have made ultrafast pulsed laser ablation an emerging technology of precise laser machining and material deposition.